Guy, many thanks for your help. The agents did indeed lock down my account until it could get sorted out. I now have my number back, access to my giffgaff account online, and all of my other accounts that the hacker used my phone to bypass security measures on.
just 15 minutes ago I received an email showing someone is trying once again to reset my giffgaff password. I assume the same person.
It would make me way more comfortable if 2FA was an option for logging in to giffgaff.